Carboxylic acids are useful feedstocks for the production of alkenyl carboxylates. Thus, for example, acetic acid is used to manufacture vinyl acetate which is generally prepared commercially by contacting ethylene and acetic acid with molecular oxygen in the presence of a catalyst active for the production of vinyl acetate. Suitably, the catalyst may comprise palladium, an alkali metal acetate promoter and an optional co-promoter (for example, gold or cadmium) on a catalyst support. Acetic acid may be produced by the catalytic oxidation of ethylene and/or ethane.
Integrated processes for producing acetic acid and/or vinyl acetate are known in the art. EP-A-0 877 727 discloses an integrated process for the production of acetic acid and/or vinyl acetate in any pre-determined and variable proportions from a gaseous feedstock comprising ethylene and/or ethane. The integrated process comprises a first step wherein ethylene and/or ethane is catalytically oxidised in a first reaction zone to produce a first product stream comprising acetic acid, water and ethylene and optionally ethane, carbon monoxide, carbon dioxide and/or nitrogen. The acetic acid and ethylene produced in this first reaction zone are then contacted in a second reaction zone with a molecular oxygen-containing gas in the presence of a catalyst to produce a second product stream comprising vinyl acetate, water, acetic acid and optionally ethylene. No mention is made of any control of the production ratio of ethylene to acetic acid from the catalytic oxidation of ethane and/or ethylene.
Research Disclosure 2244 of 1992 (June) No. 338 describes a process for the oxidation of ethane and/or ethylene to produce acetic acid in which the by-product carbon monoxide is oxidised to carbon dioxide. According to this document, the acetic acid, unreacted ethane (if present) and ethylene is passed with or without carbon dioxide and water removal, to a reactor having a suitable catalyst for the production of ethyl acetate or, with the addition of oxygen, for the production of vinyl acetate. This document is silent on the control of the ratio of ethylene to acetic acid produced in the oxidation step.
In the manufacture of vinyl acetate from ethylene and acetic acid, the molor ratio of the fresh feed ethylene to acetic acid is desirably unity or approximately unity. Thus, in an integrated process in which ethane is oxidised in an oxidation reaction zone to produce ethylene and acetic acid for use in a second reaction zone for the production of vinyl acetate to maxamize overall integrated process efficiency and also vinyl acetate output, the molar ratio of ethylene to acetic acid produced in the oxidation reaction zone, is desirably unity or approximately unity depending upon the selectivity/yield in the second reaction zone.
The effect of water on the formation of acetic acid during the oxidation of ethane to acetic acid is described in U.S. Pat. No. 4,250,346, but this does not disclose the effect of ethylene on the ratio of products formed.
Thus, there remains a need for a process for the oxidation of a C2 to C4 alkane to produce the corresponding alkene and carboxylic acid in which the molar ratio of alkene to carboxylic acid produced is adjusted or maintained at a pre-determined value.
Typically, in the oxidation of an alkane such as ethane, carbon monoxide and/or carbon dioxide (the carbon oxides) are formed as by-products. The formation of high levels (typically greater than 15 mol %) of these carbon oxides is undesirable as it generally results in increased capital and production costs. A low selectivity to carbon oxides reduces the need for costly reaction and heat removal systems and removal systems for product purification, reduces the cost of carbon oxide removal systems, lowers operating costs and results in increased yields to the desirable carboxylic acid and alkene products.